Enhancing muscle capacity and lean muscle mass is a primary goal of athletic training. The ability of skeletal muscle to deliver high output correlates with the amount of phosphocreatine stored in the tissue. The fuel for muscular work in the body is adenosine tri-phosphate, or ATP. During intense exercise ATP is utilized very rapidly. The body does not store much ATP in muscle, so other substances must be broken down in order to replenish ATP used during exercise. If the ATP is not replenished, fatigue occurs and force/power production declines. Of all the substances in the body that can replenish ATP, the fastest is phosphocreatine. Creatine kinase, the enzyme responsible for synthesizing phosphocreatine from creatine and ATP, is tightly regulated and highly responsive to levels of ATP and creatine in the muscle cell. Thus, upon ATP utilization during exercise the stored phosphate in phosphocreatine is rapidly converted to ATP for additional muscle capacity.
Creatine is a natural dietary component primarily found in animal products. In the body, creatine is stored predominantly in skeletal muscle, generally in the form of phosphocreatine. Total creatine content of mammalian skeletal muscle (i.e., creatine and phosphorylated creatine) typically varies from about 100 to about 140 mmol/kg. The level of creatine and phosphocreatine present in skeletal muscle can be increased through dietary supplementation with creatine.
Creatine is taken up into tissues, such as skeletal muscle, by means of an active transport system using the receptors CRT1 and CRT2. Zorzanzo, A, et al., Biochem. J., 2000; 349:667-688, the contents of which are incorporated herein by reference. Regulation of CRT1 and CRT2 are not fully elucidated. Some studies identified insulin as a possible regulator of creatine uptake. In a study by Stengee et al., insulin was co-infused along with creatine supplementation. (Am. J. Physiol., 1998; 275:E974-79). Insulin was shown to enhance creatine accumulation in muscle, but only at extremely high or super-physiological concentrations. A previous study by Green et al. involved ingestion of creatine in combination with a carbohydrate-containing solution to increase muscular uptake of creatine by creating physiologically high plasma insulin concentrations. Green et al. reportedly found the quantity of carbohydrate necessary to produce a significant increase in creatine uptake, as compared to creatine supplementation alone, was close to the limit of palatability.
Insulin administration itself is also a detrimental method of improving creatine levels in muscle. Insulin has both positive and negative functions in human physiology. For example, insulin affects fat mass by enhancing the deposition of fatty acids leading to increased adiposity. For those seeking gains in lean body mass, fat mass is negatively correlated with physique enhancement.
Thus, a need exists for processes and compositions that promote improved creatine transport into muscle cells that are agreeable to the subject.